powerpc/mm: Respect _PAGE_COHERENT on classic ppc32 SW

Since we now set _PAGE_COHERENT in the Linux PTE we shouldn't be clearing
it out before we setup the SW TLB.  Today all the SW TLB machines
(603/e300) that we support are non-SMP, however there are some errata on
some devices that cause us to set _PAGE_COHERENT via CPU_FTR_NEED_COHERENT.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>